Process for the preparation of urea

ABSTRACT

A process for the preparation of urea from ammonia and carbon dioxide at an elevated temperature and pressure having a reaction zone and a stripping zone. In the reaction zone, carbon dioxide and a portion of the ammonia are converted to ammonium carbamate, and a portion of the ammonium carbamate is converted to urea, the combined conversions resulting in a net formation of heat. In the stripping zone, a urea product stream containing unconverted ammonium carbamate is heated by heat exchange with the reaction zone to decompose a portion of the ammonium carbamate. In the reaction zone, the conversion of ammonium carbamate into urea is continued until the quantity of urea formed is at least 50 percent of that quantity of urea that would be obtained at equilibrium under the reaction conditions present in the reaction zone.

This invention relates to a process for preparing urea from ammonia andcarbon dioxide at elevated temperatures and pressures whereby the netheat produced by the urea synthesis reaction can be more efficiently andeffectively utilized.

One process for the preparation of urea which has found wide use inpractical applications is described in European Chemical News UreaSupplement of Jan. 17, 1969, at pages 17-20. In the process theredisclosed, the urea synthesis solution is formed in a reaction zonemaintained at a high pressure and temperature, and is thereaftersubjected to a stripping temperature at the synthesis pressure byheating this solution and contacting it countercurrently with a carbondioxide stripping gas so as to decompose a major portion of the ammoniumcarbamate contained therein. The gas mixture thus formed, containingammonia and carbon dioxide together with the stripping gas and a smallquantity of water vapor, is removed from the remaining urea productstream and introduced into a condensation zone wherein it is condensedto form an aqueous ammonium carbamate solution. This aqueous carbamatesolution, as well as the remaining non-condensed gas mixture, isrecycled to the reaction zone for conversion to urea. The condensationof this gas mixture returned to the reaction zone provides the heatrequired for the conversion of ammonium carbamate into urea, and no heatneed be supplied to the reaction zone from the outside.

The heat required for the stripping treatment is provided by thecondensation of high-pressure steam on the outside of tubes in avertical heat exchanger in which the stripping takes place. According tothis publication, approximately 1,000 kg of steam at a pressure of about25 bar is required per ton of urea. In practice, the consumption of highpressure steam (25 bar) has been reduced to about 850 kg per ton ofurea, and the heat used in the stripping treatment can be partiallyrecovered by condensation of the resulting gas mixture. Thiscondensation, however, takes place at a relatively low temperature levelwith the result that steam of only 3-5 bar is produced, for which thereis relatively little use in either this process or outside the process.For this reason, and particularly in view of continually increasingenergy prices, it is highly desirable to reduce the consumption ofhigh-pressure steam as much as possible and, depending on localconditions and needs, it may be desirable as well to avoid or greatlyreduce the production of surplus low-pressure steam.

Various proposals have already been made to use the heat released in theformation of carbamate from ammonia and carbon dioxide to provide atleast a part of the heat requirements in the stripping treatment. Forvarious reasons, however, these proposals have not found practicalapplication. For example, in British patent specification No. 1,147,734,a process is disclosed wherein the ammonium carbamate and urea synthesisis effected in two successive reaction zones. Fresh ammonia and at leasta portion of the fresh carbon dioxide are fed into the first reactionzone, and the heat released by the formation of ammonium carbamate inthis first reaction zone is transferred, via builtin heat exchangeelements, to the urea synthesis solution formed in the second reactionzone at a pressure equal to or lower than that in the first reactionzone, resulting in the decomposition of unreacted ammonium carbamate.The decomposition products thus formed are stripped from the remainingurea solution by means of an inert gas.

In carrying out this known process, the fresh ammonia and carbon dioxidemust be pre-heated to a high temperature, for instance to the synthesistemperature maintained in the first reaction zone, in order to havesufficient heat to effect the decomposition of ammonium carbamate in thestripping zone. This pre-heating of the reactants consumes a significantquantity of high-pressure steam. Of the surplus heat formed in the ureasynthesis, however, only a small portion can be recovered as steam at apressure of about 5 bar via a cooling system for the second reactionzone. A larger portion of this surplus heat is recovered at a relativelylow temperatures level in the absorption column, operating at thepressure of the second reaction zone, in which column the ammonia andcarbon dioxide stripped from the urea product stream are separated fromthe inert stripping gas. The steam so produced has a relatively lowpressure of only about 2 bar, for which there is little use within theprocess.

In another process as disclosed in U.S. Pat. No. 3,957,868, the ureasynthesis reaction is carried out in the shell side of a verticaltubular heat exchanger wherein the tops of the tubes open into theshell-side space of the heat exchanger. The urea synthesis solutionformed in the shell-side reaction zone flows into and down the inside ofthe tubes wherein ammonium carbamate is decomposed by means of the heatof reaction transferred through the tube walls, and the ammonia andcarbon dioxide thus released are stripped from the solution.

In order to maximize the decomposition of ammonium carbamate, it isnecessary to have the heat of reaction available at as high atemperature level as possible, and to insure optimum heat transfer tothe solution to be stripped. The measures proposed in this reference toaccomplish this include carrying out the urea synthesis and thestripping at a temperature of 210° to 245° C. and a pressure of 250 to600 atmospheres, maintaining a relatively high NH₃ /CO₂ molar ratio inthe liquid phase in the reaction zone, and recirculating a part of thegas mixture removed from the stripping zone into the bottom of thereaction zone together with an extra quantity of gas mixture which iskept available for recirculation by condensing only a portion of thegaseous ammonia and carbon dioxide supplied to the reaction zone toammonium carbamate. Thus, the weight of urea produced will be lowrelative to the quantities of ammonia and carbon dioxide supplied to thereaction zone.

Because of the high temperatures employed in this process, the materialsof construction to be used must satisfy high standards of corrosionresistance. Moreover, a relatively high capital investment is requiredfor the equipment because of the higher pressure used, and the specialdesign of the combined reactor-stripper. When all of these factors aretaken into account, this known process, in practical application, haslittle or no economic advantage over the process described in theEuropean Chemical News Urea Supplement which has found generalapplication.

It is an object of the present invention to provide a process for theproduction of urea wherein the quantity of high-pressure steam requiredfor the decomposition of non-converted ammonium carbamate issubstantially reduced while avoiding the disadvantages of theabove-mentioned known processes.

In accordance with the invention, the condensation of ammonia and carbondioxide in the reaction zone is effected in the presence of relativelylarge quantities of urea and water, which causes the average temperaturein the reaction zone to rise considerably. The heat of reaction from thereaction zone is exchanged with the stripping zone wherein it isutilized to decompose ammonium carbamate in the product stream. Whencarried out according to the invention the heat of reaction from thereaction zone is available at a high temperature level and can be moreeffectively and efficiently utilized than in the prior art in thedecomposition and stripping of ammonium carbamate in the urea productstream.

Specifically, the invention relates to a process for preparing urea fromcarbon dioxide and an excess amount of ammonia at an elevatedtemperature and pressure, wherein at least a part of the urea synthesisis effected in a reaction zone which is in heat exchange with astripping zone wherein ammonium carbamate is decomposed and thedecomposition products are removed from a urea product stream, and thestripped urea product stream is processed to a urea solution or solidurea. In accordance with the present invention, the urea synthesistaking place in the reaction zone exchanging heat with the strippingzone is continued until the quantity of urea produced is at least 50percent of the equilibrium quantity of urea obtainable under thereaction conditions prevailing in the reaction zone. Preferably, theurea synthesis in the reaction zone is continued to the point that thequantity of urea produced is at least 70 percent of that which would beformed at equilibrium.

In order to further increase and make maximum use of the heat releasedin this reaction zone, it is desirable to insure intensive mixing of thecontents of the reaction zone, which has the effect of raising thetemperature level and promotes an increased transfer of heat to thestripping zone. To this end, the stripping can be carried out in a knownmanner within the tubes of a vertical heat exchanger while the shellside of the exchanger serves as the reaction zone, and the contents ofthe reaction zone are sufficiently mixed so that the difference intemperature between the top and bottom of the reaction zone is limitedto 5° C. at most. Preferably, to maximize the heat transfer, thistemperature difference should be maintained at most at about 2° C.

Contrary to the above-noted process disclosed in U.S. Pat. No.3,957,868, the present process accomplishes an effective and efficientexchange of heat from the reaction zone to the stripping zone withoutthe need for extremely high pressures and temperatures, but rather undermore usual urea synthesis pressure and temperature conditions, such as125-250 bar and 170°-205° C., respectively. It is also possible whenusing this process to maintain the stripping zone and the reaction zoneat the same pressure, although enhancement of the heat transfer andstripping effect may be realized by operating the stripping zone at apressure lower than the reaction zone.

The amount of ammonium carbamate which is still present in the ureasynthesis solution formed in the reaction zone can be decreased by usinga large excess of ammonia and longer residence times, so that a largerproportion of the ammonium carbamate formed is converted to urea. Thisrequires, however, a larger reaction zone per unit of urea capacity,which in view of the design capacities of modern urea synthesis plantsof 1500-2000 tons per day or more, may give rise to constructional andlogistic problems.

This dilemma can be avoided by permitting the urea synthesis solutionformed in the reaction zone to continue reacting in a separateafter-reaction zone until, depending on the reaction conditionsemployed, 90 to 97 percent of the equilibrium amount of urea that can beachieved under these conditions has been formed. Since the rate ofconversion of carbamate to urea decreases as equilibrium is approachedcontinuing the urea synthesis under the same conditions as used in thereaction zone would require a long residence time and therefore a largeafter-reaction zone. The additional cost could then outweigh theadvantage of a small increase in the amount of urea produced. However byhaving the urea synthesis in the after-reaction zone, and possibly inthe reaction zone as well, take place at a pressure which is higher thanthe pressure at which the stripping treatment takes place, thecarbamate-urea equilibrium shifts toward the urea side with the resultthat the urea synthesis solution contains less carbamate that has to bedecomposed. Even a small increase of pressure in the reaction zones willresult in higher conversion, but if a higher pressure is used in theafter-reaction zone than in the reaction zone, a pressure differentialof 10 bar or more is advisable. In that case, a pressure of 125-150 baris preferably used in the reaction zone, and a pressure of 160-250 baris preferably used in the after-reaction zone, and the strippingtreatment can then take place at the pressure used in the reaction zone.

If the amount of heat available in the reaction zone for transfer to thestripping zone is insufficient, due to too low a temperature level, toachieve an acceptable level of carbamate decomposition and expulsion ofthe gases thus formed, the remaining urea product stream leaving thestripping zone, still containing ammonium carbamate, can be subjected toa second stripping treatment in which the heat required fordecomposition of the carbamate is supplied by means of steam. Thissecond stripping treatment can be effected at the pressure maintained inthe first stripping treatment, or a lower pressure may be used.Alternatively, it is possible to treat only a portion of the ureasynthesis solution in the stripping zone exchanging heat with thereaction zone, while the remainder is sent directly to a parallelstripping treatment wherein the heat is provided by steam. It is alsopossible to first strip the urea synthesis solution using steam heat andto subsequently introduce it into the stripping zone exchanging heatwith the reaction zone.

When carrying out the preparation of urea in accordance with theinvention, it is possible to increase the amount of heat exchanged fromthe reaction zone to the stripping zone by maintaining the pressure inthe reaction zone higher than the stripping zone. As the pressure in thereaction zone increases, the condensation of carbon dioxide and ammoniato ammonium carbamate takes place at higher temperatures and,consequently, more heat will be available for the decomposition ofcarbamate in the stripping zone. Preferably, a pressure differentialbetween the heat-exchanging reaction and stripping zones should be atleast about 20 bar. On the other hand, a pressure difference of morethan 120 bar, although theoretically possible, complicates the design ofthe reactor-stripper apparatus. Most preferably, the reaction zoneshould be maintained at a pressure of 40 to 60 bar higher than in thestripping zone. This provides a suitable driving force for the heatexchange without the need for equipment of a more complex design, andpermits the compression of the gas released in the stripping zone to thereaction zone pressure without the need for intermediate cooling.

The invention will be explained in greater detail with reference to theattached drawings.

FIG. 1 illustrates an embodiment of the process according to theinvention wherein the urea synthesis solution is stripped in twosuccessive steps, the first stripping step deriving heat from heatexchange with the reaction zone, and the second deriving heat fromsteam.

FIG. 2 illustrates an embodiment of the invention in which a portion ofthe urea synthesis solution is stripped utilizing heat derived from heatexchange with the reaction zone, and a remaining part of the ureasynthesis solution is subjected to a separate stripping treatmentwherein the heat is derived from steam.

FIG. 3 illustrates another embodiment of the invention wherein a firstminor portion of the carbamate present in the urea synthesis solution isdecomposed and expelled by heating with steam, and a major portion ofthe remaining ammonium carbamate still present in the urea solution isstripped with the aid of heat derived from heat exchange with thereaction zone.

In each of the figures, the equipment identified with the letter Arepresents a reactor-stripper which is here illustrated, for example, asa vertical shell and tube heat exchanger wherein the shell side of theheat exchanger is the reaction zone and the tube side of the exchangeris the stripping zone. Furthermore, in each of the drawings, Brepresents a high pressure carbamate condenser, C represents anafter-reactor, D represents a heater-decomposer, E represents ascrubber, F represents a carbon dioxide compressor, G represents anammonia heater, H represents a gas ejector, K represents an ammoniapump, and L represents a carbamate pump.

Referring first to the embodiment illustrated in FIG. 1, the ureasynthesis solution obtained from after-reactor C, which containsnon-converted ammonium carbamate and free ammonia in addition to ureaand water, is passed via line 20 into the stripping zone ofreactor-stripper A wherein it is heated by heat exchange with thereaction zone and passed countercurrently against gaseous carbondioxide. This carbon dioxide is introduced into the bottom of thestripping zone via line 1, carbon dioxide compressor F, and line 2 at apressure of, for example, 240 bar. Air or some other oxygen-containinginert gas mixture is added to the carbon dioxide in order to maintainthe stainless steel materials of construction contacting thecarbamate-containing solutions at high temperatures in a passive state.

In the stripping zone, the heat derived from the reaction zone causesthe decomposition of ammonium carbamate to form a gas mixture consistingof ammonia, carbon dioxide, and water vapor which is expelled from theurea synthesis solution by the stripping treatment, and, together withthe freshly supplied carbon dioxide, is discharged from the strippingzone of reactor-stripper A through line 4. The residual solutioncontaining product urea, which still contains an amount of ammoniumcarbamate, is discharged from the stripping zone through line 5,expanded in expansion valve 6 to a pressure of, for example, 140 bar orlower, and introduced into gas-liquid separator S₁ wherein a gas mixtureof ammonia, carbon dioxide, and water vapor evolved from the expansionis separated from the remaining urea product solution. This remainingurea product is sent through line 7 to heater-decomposer D, hereillustrated as a vertical tubular heat exchanger, in which heat issupplied by means of 20-25 bar steam. In heater-decomposer D,substantially all of the remaining ammonium carbamate still present inthe urea product stream is decomposed into ammonia and carbon dioxideand, together with a small amount of water vapor, is discharged as a gasmixture through line 8. The urea product solution is discharged throughline 9 after which it can be subjected to further known and customarytreatments to form a concentrated urea solution or solid urea.

In the embodiment illustrated in FIG. 1, the urea product solution to betreated in the second stripping step is introduced into the bottom ofheater-composer D and the solution and evolved gases flow through thetubes cocurrently. It is also possible to feed to urea solution to theheater from the top and to let it flow downward in the tubes,countercurrent to the expelled gases. In the latter case, the separationof the gas mixture released by the expansion in expansion valve 6 cantake place in the head of heater-decomposer D, and a separate gas-liquidseparator S₁ is not necessary.

A portion of the gas mixture discharged from the stripping section ofreactor-stripper A through line 4 is led into the bottom of shell sideof reactor-stripper A in which the reaction zone is located, via line 10and ejector H. Ejector H is driven by fresh ammonia, which is suppliedthrough line 3, brought to the desired pressure by pump K and thedesired temperature in ammonia heater G and thereafter into ejector Hthrough line 11.

The remaining part of the gas mixture discharged from the stripping zonethrough line 4 is introduced into carbamate condenser B via line 12 andexpansion valve 13. Ammonium carbamate condenser B is maintained at thesame pressure as heater-decomposer D. Carbamate condenser B is also fed,through lines 8 and 15, with the gas mixture expelled fromheater-decomposer D and possibly, through lines 14 and 15, with the gasmixture separated from the urea solution in gas-liquid separator S₁,where the separator is employed. These gas streams fed to carbamatecondenser B are partially condensed in the presence of the carbamatesolution supplied through line 16 from scrubber E, and the heat ofcondensation can be recovered and used to produce steam of, for example,3-6 bar. The carbamate solution thus formed is brought up to thepressure of the reaction zone by means of carbamate pump L, and isintroduced via line 17 into the bottom of the reaction zone ofreactor-stripper A. The non-condensed gas mixture from carbamatecondenser B, which contains part of the inert gases introduced into theprocess with the fresh carbon dioxide and ammonia, flows to scrubber Ethrough line 18.

In the reaction zone of reactor-stripper A, a large portion of theammonia and carbon dioxide fed through lines 10 and 11 via ejector H isconverted to ammonium carbamate, and a portion of this ammoniumcarbamate and the carbamate supplied from carbamate condenser B throughcarbamate pump L and line 17 are converted to urea and water. Thecondensation reaction forming ammonium carbamate is exothermic, formingan amount of heat, whereas the conversion of carbamate to urea and wateris endothermic, consuming heat. However, a surplus of heat is producedby the combined reactions and is available for the decomposition ofnon-converted carbamate in the stripping zone of reactor-stripper A. Byhaving the formation of carbamate take place in the presence of anamount of urea and water in accordance with the invention, a highertemperature is reached in the reaction zone, so that a greater portionof the surplus heat can be transferred to the stripping zone. Thiseffect is already appreciable when, in the reaction zone, about one-halfof the equilibrium amount of urea obtainable under the reactionconditions employed is formed, and the effect will be stronger as theurea conversion more closely approaches equilibrium. For instance, if ina typical embodiment of the process according to FIG. 1 at a pressure ofabout 240 bar, molar ratio of ammonia to carbon dioxide of 3.40:1 and amolar ratio of water to carbon dioxide of 0.46:1 about 50% of theequilibrium amount of urea is formed the average temperature in thereaction zone will be about 195° C. With 60, 70, 80 and 90% of theequilibrium amount of urea formed the temperature will be about 196°,199°, 201° and 203° C., respectively. The temperature increasesresulting from higher conversions of ammonium carbamate to urea andwater are substantial with respect to the temperature differentialacross the heat exchange surfaces in the reactor-stripper. Preferably,the degree of conversion of carbamate to urea in the reaction zone willbe selected to approach about 70 percent of equilibrium. Urea formationof more than 90 percent of the equilibrium amount, however, requires aconsiderably longer residence time due to the strongly decreasingreaction rate, and consequently requires an unattractively large volumein the reaction zone.

Given the pressure, the amount of excess ammonia, the molar ratio of H₂O/CO₂ and residence time, the temperature that can be reached in thereaction zone is fixed. Also, the amount of gas mixture that can be fedvia ejector H is fixed, inasmuch as it affects the residence time, andthus the amount of heat transferred cannot be further increased byincreasing the amount of this gas mixture. The heat that is not utilizedis removed from the process by condensing part of the gas mixtureremoved from the stripping zone in carbamate condenser B and scrubber E,and by condensing part of the gas mixture discharged from the reactionzone in after-reactor C and scrubber E.

The contents of the reaction zone should be mixed intensively in orderto achieve the most uniform heat distribution possible and to maximizethe heat transfer to the stripping zone. Some mixing is achieved byhaving the gas mixture and the carbamate solution flow through thereaction zone from the bottom to the top. However, this mixing, andconsequently the heat transfer to the stripping zone can be furtherimproved by the installation of baffles, guide plates, or similarelements in the reaction zone.

The aqueous urea solution containing excess ammonia and ammoniumcarbamate formed in the reaction zone, together with the non-condensedgas, is sent via line 19 to after-reactor C, which is maintained at thesame pressure as the reaction zone in reactor stripper A, and there theurea synthesis is continued until at least about 90 percent of theequilibrium amount of urea which can be obtained under the reactionconditions in the after-reactor has been formed. The heat required forthis conversion of ammonium carbamate to urea is obtained by theformation of an additional quantity of ammonium carbamate in theafter-reactor from the gaseous ammonia and carbon dioxide fed throughline 19. The urea synthesis solution thereby obtained is then led vialine 20 to the stripping zone of reactor-stripper A. The gas mixtureleaving the top of after-reactor C, consisting of inert gases, ammonia,carbon dioxide, and water vapor, is sent via line 21 and expansion valve22 to scrubber E, wherein the ammonia and carbon dioxide are recoveredat the same pressure used in heater-decomposer D and carbamate condenserB by scrubbing with water or a dilute solution of ammonium carbamatesupplied through line 23, while removing the heat of absorption. Theresidual off-gas mixture remaining after scrubber E is dischargedthrough line 24, and the carbamate solution obtained in scrubber E isintroduced into carbamate condenser B through line 16.

When carrying out the embodiment of the invention as illustrated in FIG.1 and described above, a reduction in high-pressure steam consumption(25 bar) of about 10 percent relative to the process described inEuropean Chemical News Urea Supplement and discussed above, can beachieved, the precise steam savings depending on the process conditionschosen. In this embodiment, the total amount of urea synthesis solutionis treated in the stripping zone of reactor-stripper A. However, theamount of heat available for the stripping treatment is limited, so thatonly a relatively low stripping efficiency can be achieved, and aconsiderable amount of carbamate still remains to be decomposed andexpelled in heater-decomposer D.

In the embodiment illustrated in FIG. 2, the urea synthesis solutionformed in after-reactor C is divided into two portions. One portion, forexample 20 to 50 percent of the total amount, is fed to the strippingzone of reactor-stripper A through line 20, and the remaining portion,for example 50-80 percent of the total amount, is carried through line25 to expansion valve 26 and thereafter to gas-liquid separator S₁ forremoval of the gas mixture evolved during the expansion, and theremaining urea synthesis solution is introduced into heater-decomposerD. The quantity of the urea synthesis solution from after-reactor Cdirectly fed to the stripping zone in reactor-stripper A is such that anormal or higher than normal stripping efficiency can be achieved,taking into account the amount of heat available for the strippingtreatment, so that after expansion in expansion valve 6, the resultingurea product stream can be further processed in a known manner togetherwith that portion of the urea product stream leaving heater-decomposerD. In this case, the total amount of ammonia needed in the process isfed into the reaction zone of stripper-reactor A. If necessary, however,a portion of the fresh ammonia can be fed into carbamate condenser Bthrough line 39 and expansion valve 40.

In practicing the embodiment illustrated in FIG. 2 as described above, areduction in high-pressure steam consumption (25 bar) of about 20-60percent can be achieved relative to the process described in EuropeanChemical News Urea Supplement and discussed above, again the precisesteam savings depending upon the process conditions chosen.

In the embodiment illustrated in FIG. 3, the urea solution dischargedfrom after-reactor C is first treated in heater-decomposer D with steamto decompose an initial quantity of unconverted ammonium carbamate. Inthis embodiment, heater-decomposer D is schematically shown as acocurrent heater-decomposer, but a countercurrent heater-decomposer canbe used as well. To protect the materials of construction fromcorrosion, air or an oxygen-containing inert gas can be supplied toheater-decomposer D through line 38 for passivation.

In heater-decomposer D, a portion of the ammonium carbamate isdecomposed and removed from the urea synthesis stream and a portion ofthe gas mixture thus formed is returned through line 27 to after-reactorC. The remaining portion of this gas mixture is discharged to carbamatecondenser B via line 35 and expansion valve 36. In this embodiment,after-reactor C is maintained at a higher pressure than the reactionzone of reactor-stripper A, the latter being maintained at a pressure of125-200 bar, for example, 140 bar, and the after-reactor beingmaintained at a pressure of 160-250 bar, for example, 190 bar.

The urea product stream leaving heater-decomposer D, still containing aportion of ammonium carbamate, is passed via line 28 and throughexpansion valve 29 into gas-liquid separator S₂, which operates at thepressure of the stripping zone of reactor-stripper A, wherein the gasmixture evolved from the expansion is separated from the urea productstream and introduced into carbamate condenser B through line 30. Theremaining urea solution, still containing a portion of the non-convertedcarbamate and free ammonia, is introduced via line 31 into the strippingzone of reactor-stripper A. Here, substantially all of the ammoniumcarbamate still present in the urea solution is decomposed with the aidof heat transferred from the reaction zone so that the urea solutiondischarged through line 5 has only a small amount of carbamateremaining, which can be removed in a known manner.

In this embodiment, the gas mixture containing non-condensed inertcomponents leaving the reaction zone of reactor-stripper A through line19 is separated from the urea solution in gas-liquid separator S₃, andthe remaining urea solution is led via line 33 to pump M whereby it isbrought up to the pressure of after-reactor C is introduced therein vialine 34. The gas mixture withdrawn from gas-liquid separator S₃ is ledvia line 32 to carbamate condenser B wherein the ammonia and carbondioxide are condensed out. The inert-containing gases from carbamatecondenser B, via line 37, as well as the inert gases from after-reactorC, via line 21 and expansion valve 22, are discharged to scrubber E inwhich ammonia and carbon dioxide still present in the inert gases arerecovered by scrubbing with a dilute carbamate solution supplied throughline 23.

In the embodiment illustrated in FIG. 3 and described above, the ureasolution from the reaction zone of reactor-stripper A continues to reactin after-reactor C at a higher pressure than in the reaction zone, sothat a higher degree of conversion of ammonium carbamate to urea isachieved, and thus a smaller amount of carbamate need be decomposed. Itis also possible, however, to carry out the after-reaction at the samepressure used in the reaction zone. Although a larger quantity ofcarbamate will then have to be decomposed there is the advantage thatseparators S₂ and S₃ and pump M, together with associated lines, valves,and measuring and control instrumentation, can be dispensed with.

EXAMPLE

Urea was prepared in accordance with the process illustrated in FIG. 2as described above. For each tonne of urea to be produced, 567 kg ofammonia at a temperature of 120° C. is fed to the reaction zone ofreactor-stripper A through ammonia pump K and ammonia heater G, and 733kg carbon dioxide and 29 kg of inert gases (primarily air) are fed tothe stripping zone of reactor-stripper A through carbon dioxidecompressor F. The pressure in both zones of reactor-stripper A ismaintained at 240 bar. The reaction zone is additionally fed, viaejector H, with a gas mixture consisting of 212 kg ammonia, 651 carbondioxide, 13 kg water, and 23 kg inert gas and, via carbamate pump L,with 1584 kg of a carbamate solution consisting of 818 kg ammonia, 562kg carbon dioxide, and 204 kg water. The volume of the reaction zone,and consequently the residence time, is such that, with the pressure of240 bar therein maintained and the temperature of about 200° C.associated with this pressure, a gas-liquid mixture is formed in whichthe liquid phase is composed of 800 kg urea, 1009 kg ammonia, 524 kgcarbon dioxide, and 449 kg water, and the gas phase consists of 135 kgammonia, 102 kg carbon dioxide, 8 kg water vapor, and 23 kg of inertcomponents.

The quantity of urea formed in the reaction zone is about 76% of thequantity of urea that would be obtained if the conversion would beallowed to proceed to equilibrium.

This gas-liquid mixture is introduced into after-reactor C via line 19for further reaction wherein a urea synthesis solution consisting of1000 kg urea, 913 kg ammonia, 393 kg carbon dioxide, and 510 kg of wateris formed. About 32.5 wt.% of this urea synthesis solution is sent tothe stripping zone of reactor-stripper A via line 20, while theremaining portion, after expansion to a pressure of 140 bar, is fed togas-liquid separator S₁.

In the stripping zone, a major portion of the carbamate present in theurea solution fed thereto is decomposed. The ammonia and carbon dioxidethereby released are expelled by stripping with carbon dioxide fed intothe stripping zone through line 2, resulting in a discharge of 1124 kgof a gas mixture through line 4 consisting of 265 kg ammonia, 814 kgcarbon dioxide, 16 kg water vapor, and 29 kg inert gas. About 80 wt.% ofthis gas mixture is introduced into the reaction zone ofreactor-stripper A via line 10, and the remaining portion, afterexpansion to 140 bar, is sent to carbamate condenser B. The remainingurea product stream discharged from the stripping zone ofreactor-stripper A contains, in addition to 325 kg urea and 150 kgwater, only 32 kg ammonia and 47 kg carbon dioxide.

In gas-liquid separator S₁, 184 kg of gas mixture evolved from theexpansion of the urea synthesis solution from after-reactor C from 240to 140 bar is separated from 1716 kg of remaining urea product solution.This urea product solution, consisting of 675 kg urea, 494 kg ammonia,and 213 kg carbon dioxide, the balance being water, is heated to 210° C.in co-current heater-decomposer D resulting in the decomposition of anadditional quantity of ammonium carbamate, and the evolution of ammoniaand carbon dioxide in gaseous form. The resulting gas mixture contains268 kg ammonia, 163 kg carbon dioxide and 30 kg of water. The remainingurea product solution leaving heater-decomposer D consists of 675 kgurea, 304 kg water, 226 kg ammonia, and 50 kg carbon dioxide. The heatrequired for the decomposition and removal of off-gases inheater-decomposer D is provided by 354 kg of superheated steam having atemperature of about 300° C. and a pressure of 26 bar. From thecondensation taking place in carbamate condenser B of the gas mixturessupplied thereto from the stripping zone, gas-liquid separator S₁ andheater-decomposer D, sufficient heat is released to form 550 kg ofsaturated steam of 3.5 bar.

We claim:
 1. Process for the preparation of urea from ammonia and carbondioxide at an elevated temperature and pressure having a reaction zoneand a stripping zone whereinin said reaction zone, carbon dioxide and aportion of said ammonia are converted to ammonium carbamate, and aportion of said ammonium carbamate is converted to urea to form areaction zone effluent containing product urea, unconverted ammoniumcarbamate, and excess ammonia, said conversions resulting in a netformation of heat, and in said stripping zone, a urea product streamcontaining unconverted ammonium carbamate is heated to decompose atleast a portion of said ammonium carbamate by heat exchange with saidreaction zone, and stripped with carbon dioxide to remove gaseousammonia and carbon dioxide thus formed from said urea productstream,characterized in that said reaction zone is maintained at apressure of at least 20 bar higher than the pressure in said strippingzone and the conversion of ammonium carbamate to urea in said reactionzone is continued until the quantity of urea formed is at least 50percent of that quantity of urea that would be obtained at equilibriumunder the reaction conditions present in said reaction zone.
 2. Processaccording to claim 1, characterized in that the conversion of ammoniumcarbamate into urea in said reaction zone is continued until thequantity of urea formed is at least 70 percent of that quantity of ureathat would be obtained at equilibrium under the reaction conditionspresent in said reaction zone.
 3. Process according to claim 1 or 2,characterized in that the contents of said reaction zone are intensivelymixed.
 4. Process according to claim 3, characterized in that saidreaction and stripping zones are within a vertical tube and shell heatexchanger, said stripping zone being within the tubes of said heatexchanger and said reaction zone being within the shell of said heatexchanger, and wherein the temperature differential between the top andbottom of said reaction zone is limited to at most 5° C.
 5. Processaccording to claim 4, characterized in that the temperature differentialbetween the top and bottom of said reaction zone is limited to at most2° C.
 6. Process according to claim 1, characterized in that saidreaction zone is maintained at a pressure in the range of between about125 and 250 bar and at a temperature in the range of between about 170°and 205° C.
 7. Process according to claim 1 characterized in thatpressure in said reaction zone is 40-60 bar higher than the pressurewithin said stripping zone.
 8. Process according to claim 1,characterized in that the reaction zone effluent is introduced into anafter-reaction zone wherein an additional portion of ammonium carbamateis converted to urea to form a urea product stream containing urea, in aquantity at least 90 percent of the quantity of urea that would beformed at equilibrium under the conditions prevailing in saidafter-reaction zone.
 9. Process according to claim 8, characterized inthat the pressure in said after-reaction zone is higher than thepressure in said reaction zone.
 10. Process according to claim 9,characterized in that a pressure of between about 125 and 200 bar ismaintained in said reaction zone, and a pressure of between about 160and 250 bar is maintained in said after-reaction zone.
 11. Processaccording to claim 8, characterized in that the urea product stream fromsaid after-reaction zone is heated to a temperature of between about180° and 210° C. whereby a portion of said unconverted ammoniumcarbamate is decomposed, the gas mixture thereby evolved is separatedfrom the residual product urea stream, and the residual urea productstream still containing unconverted ammonium carbamate and free ammoniais introduced into said stripping zone.
 12. Process according to claim1, characterized in that the urea product stream from said strippingzone is introduced into a second stripping zone wherein additionalammonium carbamate is decomposed and removed from the urea productstream.
 13. Process according to claim 8, characterized in that only aportion of the urea product stream from said after-reaction zone isintroduced into said stripping zone, and a remaining portion of the ureaproduct stream from said after-reaction zone is introduced into a secondstripping zone wherein ammonium carbamate is decomposed.
 14. Processaccording to claim 12, characterized in that said second stripping zoneis maintained at a lower pressure than said stripping zone.